The present invention relates to a coating method and apparatus employing an electrostatic field, in general, and to such a method and apparatus for electrostatically depositing a layer of coating material on the surface of a moving web, in particular.
Apparatus employing an electrostatic field for depositing coating materials on the surface of a moving web are well known in the prior art. In one arrangement described in U.S. Pat. No. 2,685,536 to Starkey et al, for example, a method of electrostatically depositing coating material on the surface of an object is disclosed. In said Starkey et al patent, an electrostatic field is established between a coating applicator coupled to a source of coating material and an object surface spaced therefrom. In this particular coating arrangement the electrostatic field both atomizes coating material oozing from applicator orifices and subsequently deposits same on said object surface. A more typical use of an electrostatic field in a coating application is that described in U.S. Pat. No. 4,457,256 to Kisler et al. In said Kisler et al patent an electrostatic field is employed to assist in depositing coating material on the surface of a moving web spaced therefrom after the coating material is extruded through an applicator opening. The electrostatic field assists in transporting the coating material between the applicator and web surface and, in addition, increases surface tension to thereby enable larger and therefore less critical spacing dimensions to be employed between applicator opening and the web surface during the web coating process.
In some coating arrangements, the width of a layer of coating material that can be deposited on the surface of a moving web or other substrate by a particular coating applicator may be substantially larger or smaller than a coating layer width that is optimum for some particular web coating application. In addition, in certain circumstances, it may be advantageous to provide a border or an uncoated web surface portion between the edge of a layer of coating material deposited on a web surface and the edge of the web on which the layer of coating material is deposited. As a consequence, it necessarily follows that coating width size may have to be changed (increased or decreased) if the optimum or desired coating layer width is to be achieved for the particular web coating application. Heretofore, the width of a coating layer was normally changed by changing the coating applicator configuration. In, for example, an extrusion type coating applicator, this would entail a change in the width of the opening through which coating material is extruded. However, substituting one extrusion-type coating applicator for another in order to form a particular coating layer width is relatively costly both in terms of the additional applicator or applicators that must be stockpiled in order to make such substitutions possible and the time required to physically substitute one applicator for another.
In addition to these coating layer width considerations, electrostatically depositing coating material on a web surface with coating apparatus such as those mentioned above produces a web coating layer with a varying (tapering) edge thickness. In applications where constant coating layer thickness is essential for proper functioning of the coated web in a particular operational environment, the varying thickness edge portions of the coated web would be unuseable. This varying thickness edge problem is often compensated for by coating the web or other substrate with a layer of coating material that is substantially wider than necessary or wide enough to position these edge portions in a location where they will have no effect on the application where the particular coating layer is to be employed. Disadvantages associated with this particular solution are the necessary reduction in border width between the coating layer and the edge of the coated web, which reduces the amount of border area that is available for other purposes, and the increased costs associated with the additional coating materials needed to so increase coating layer width.
It is a primary object of the present invention, therefore, to provide a coating method and apparatus that are capable of readily changing the width of a layer of coating material to be deposited on a substrate.
It is another object of the present invention to provide a coating method and apparatus that are capable of readily forming a border of any width within a range of border widths between the edge of a layer of coating material deposited on a substrate and the edge of the substrate on which the coating material is deposited.
It is a further object of the present invention to provide a coating method and apparatus that are capable of applying a layer of coating material to a substrate having edge portions that are of uniform thickness.
Other objects, features and/or advantages of the present invention will be apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.